Hot air furnaces and hot water tanks are well known in the art. Hot air furnaces and hot water tanks are typically considered to be separate fields of art, but there have been instances in which hot air furnaces have been used for generating hot water, and still other instances where hot water tanks have also been used for generating heated air.
Prior art hot air furnaces include furnaces fired by gas, such as natural gas or liquid propane gas, as well as furnaces fired by heating oil. These hot air furnaces usually are operated as forced air furnaces in which cold return air is drawn from throughout the building structure in which the furnace is installed and then passed across a heat exchanger within the furnace so that heated air exits out of the furnace and is forced through a duct system throughout the building structure. Hot water tanks, on the other hand, typically are heated by either gas, such as natural gas or liquid propane, or by electricity.
The situation thus arises that waste heat passing through the flue to the atmosphere, that is, the heat of combustion utilized to provide a source of heat for heating water in a water heater, and the heat of combustion for producing heated air in a furnace, is exhausted from the water heater or furnace through a flue and out of the building structure in which the furnace or water heater is installed. This can result in inefficiencies in that the waste heat exhausted from the structure to the atmosphere still contains a significant amount of heat energy which has not been otherwise put to a useful purpose.
Examples of earlier approaches undertaken in the effort to utilize waste heat from producing hot water can be found in U.S. Pat. No. 3,269,382 to K.M. Ronan, et al., which discloses an apparatus functioning as a combined water and space heater. An insulated housing encloses a hot water tank and air is passed through the space between the exterior surface of the hot water heater and its insulated housing, drawing heat from the exterior of the hot water tank. The heated air is then passed into the space surrounding the hot water tank.
Another attempt to provide both hot water and heated air is found in U.S. Pat. No. 2,789,769 to D. Dalin, teaching an apparatus in which water is boiled to provide a heat exchange medium for heating air to be forced throughout the structure, and having a second heat exchanger contained within the water to be boiled for heating domestic water. U.S. Pat. No. 3,333,192 to S. Bogren discloses an apparatus in which heat from a forced air furnace is utilized to heat water held in a separate hot water tank through the use of dual heat exchangers, one located in the furnace, and one located in the hot water tank.
A more recent attempt to provide an apparatus for both heating air and water is disclosed in U.S. Pat. No. 5,046,478 to Clawson, which discloses a recuperative furnace in which the heat exchange medium, water, is heated beyond its dew point and passed over a heat exchanger in which water is contained, thus providing a heated water source for both domestic hot water and for a second heat exchanger located in the plenum of the furnace over which space air is blown for heating the structure in which the furnace is installed.
None of the prior art known to the inventor discloses a system in which the waste heat in the flue gases that would normally pass from the furnace to the atmosphere is selectively directed toward a hot water tank for heating the water. Moreover, none of these devices teaches an apparatus or a method in which the water tank has a separate burner for heating water when the heat of combustion from the hot air furnace is insufficient to heat water, or when the furnace is merely used for circulating ambient or cooled air throughout the structure in which the device is installed.